The bile pigment bilirubin forms a stable complex with serum albumin and is transported in blood plasma in this form. Although the physiological significance of bilirubin binding by albumin is appreciated, very little is known about the structural features of the binding site on the protein. This proposal describes how such information might be obtained photochemically by utilizing the inherent photosensitizing action of bilirubin. The bilirubin-albumin complex will be subjected to limited irradiation with visible light in an effort to photooxidize selectively certain amino acids in the binding site before the complex dissociates. On the basis of this site - specific photooxidation, peptides containing the amino acids in question will be identified, isolated and characterized. This information would be important because it would provide insight into the mechanism of reversible interaction of bilirubin and albumin. Equally important facts of clinical significance might also emerge from this work. Newborn infants with abnormally high concentrations of serum bilirubin are often subjected to phototherapy as a means of degrading the bile pigment and preventing its entry into the central nervous system. Unchecked, this condition could result in retarded brain development and cerebral palsy. One of the potential hazards of phototherapy may be clear from the results of this project, viz. that bilirubin can sensitize the photooxidation of albumin or other important serum proteins.